honestly, any pedals are good for synths imo...you can spend more on boutique/vintage which will usually sound fantastic, but second hand, cheap pedals also get the job done (boss for example)...if there is a store near you that will accommodate, , i recommend bringing your synth in and ask to audition a bunch .... i've come across tons of pedals in recording and usually liked them all in some way.
A mutron bi-phase if you can find one, really a great phaser for keys. I've used it on piano, clavinet and rhodes, as well as pads/strings with a big smile as result. A tape echo of some sort (although hardly found in pedal form if not an emulation...) can be nice & interesting too, depending on what you want to do with the sound of course.
I have a guitarist friend who is also a good keyboard player; he has a ton of pedals for example big muff, holy grail, a boss phaser, a crybaby, some korg multieffect with delays, ring modulator etc. All of those work great on his synths, which are a combo of some VA and cheap 80s digital synths. As always, it depends on what sound you're after. Just try some pedals in a store with your synths, or buy with return policy.
If I were to buy some pedals for my synths, I'd start with a phaser of some sort. I'd also like a delay, especially if your synths don't have it already. next would be an overdrive, preferably a soft one that can add girth and some nice musical distorsion... or maybe a ringmodulator. or why not a moog murf?! go listen at their site, moogmusic.com. seems interesting. and a wahwah is highly recommended, especially if you play organ/rhodes/clavinet-like sounds. and it works really well for synth leads too. YMMV
I used to have a mutron biphase, and they rock, excellent sounding phaser.
Electro harmonix makes some expensive but wicked pedals. I'd check out analog haven, they carry super cool pedals as well. I've got a couple boss, and they work well too. Keep in mind that alot of pedals add a certain amount of noise to the signal, but in most cases it actually sounds super cool.
i not saying all pedals are great for synths, just that most pedals can do some cool stuff...ive used awesome boutique pedals .but still can have fun and get a great sound out of a boss chorus / flanger / delay / overdrive / metalzone....and most of those i picked up for less than $50....i like the cheap big muff too! i think the eh freq analyzer is just as good as a moog ring for what i want in a ring mod...i actually like it more although having an lfo would be nice. the frostwave ring mod is no joke however. i think people have their own tastes, especially when it comes to distortion, and for me, bringing my synth to a store and trying out different distortion pedals has been the way to go...if you're ever in nyc, main drag has a good selection and you can spend as long as you like.
stuff that makes weak guitar fx tends to kick ass on synths. check out those cheap digital distortion & multi fx boxes from digitech or whoever. on guitar they sound kinda cold & steril, but most synths are already that way, so the effect works. they can also be had for next to nothing these days since everyone is obsessing over all things "vintage", "analog" & non-digital.
Aside from the sound quality offered by the pedal, I'd look for something that you can easily manipulate with your fingers, being a big part of the fun. I have an old DigiTech delay that isn't too clean, but the controls are all nice and spread out so I can manipulate it manually as I play. Also nice because it has the hold/repeat on a separate button from the bypass switch. Model is PDS 1002.
I've had good luck with a Russian Big Muff on my electric piano sounds. Got an old Univox phaser that I like too, but nothing is quite like a MuTron BiPhase! I was also surprised how much I liked having the wah pedal as a foot control more than a hand control for filtering; maybe it's just because I was a guitarist first. Good luck!
IHMO, guitar pedals that don't have line-level inputs are more trouble than they are worth. I gave up on that a few years ago, and I sold a bunch of guitar stuff including an Ibanez UE405. I just got frustrated with trying to match levels and impedences, and deal with the noise and limited bandwidth of most guitar pedals.
Now I only use ones that can accommodate line ins and outs. The Boss -20 series (DD-20, CE-20, etc.) are worth looking into; they sort of bridge the gap between guitar pedals and rack/module effects. Once in a blue moon I'll run something through an old DoD FX50, but for most things I'd use it for, the Dotcom Q130 clipper/rectifier works better and is more versatile.
Z. Vex Wah Probe for anything really, super awsome wah pedal that is unique and very interactive, really expensive but theres nothing like it. All of the probe pedals are awsome, Z.Vex website has good movie demos. It's functionality is superb, entire mixes sound good through it drum machines too.
Fuzz factory (also Z.vex) - it's nasty and it requires a hot signal in, it can make an amplifier sound like it's melting, got to be careful with it's level there's nothing buffering it's output and it can destroy things.
Old boss dc-2 sounds sweet with everything i've put thru it xept for bass.
Tube screamer as mentioned, an old one or modified by analogman or R.Keley - you don't hear the difference between modified/unmod/vintage until it's cranked up.
Boss rps-10, I like it, it's an 8-bit digidelay does backward delay and pitch shifts - kinda hard to come by but they happen on ebay sometimes. Kind of tape delayish - distorts as it fades - pitch shift is nice and controlable via audio in.
Roland space echo of coarse! I sold mine a few years ago in a total pinch, my heart is still broken.
Emma discombobulator - envelope filter awsome to tweak reacts sweat to dynamics and small intervals (a D and a D# for example), does up and down sweeps - it's just sick, it likes a hot signal too.
DOD FX-17 - wah/volume pedal, cheap and unique, I don't understand how it works but I guess it's optical (no filter pot) it's different and sounds good.
Digitech whammy-1 (wh-1), every instrument sounds good through it, xpensive as hell though, the wh-2 (black model) sounds good to0 and go for 180-250.
Like I said Z.Vex will make you drool, it's pricey though.
<< another confirmed big 'nope' on the big muff. >>
good to know. Now I won't waste my time w/ that one.
Anyone using the RAT? Crystal Method supposedly used that on their first big record.
I think I might've mentioned it to you in a PM (can't remember exactly what transpired in our conversation), but I'm using a ProCo Rat for some stuff. It is very aggressive and can create some cool results, especially when tweaking a synth's resonance. It's not a "Jack-of all trades", but it does a nice job in the world of "nastiness".
Yet another "thumbs-down" on the dfegadBig Muff. I wanted to like it, but found it to sound pretty crappy on synths.
As far as non-distortion pedals go, I've gotten a lot of good use out of my MF-101 Filter, MF-103 Phaser, and Frostwave Resonator (the filter pedals sound nice when overdriven too). Once in a while I'll use an E.H. Deluxe Electric Mistress flanger as well, but not too often.