Colonel Marbles' World of Cumbustion Velocopedes, Monowheels, Penny Farthings, Anti-Gravitational Reitwagens, and sundry modes of transport..

Some people will tell you that slow is good - and it may be, on some days - but I am here to tell you that fast is better. I've always believed this, in spite of the trouble it's caused me. Being shot out of a cannon will always be better than being squeezed out of a tube. That is why God made fast motorcycles.
-Hunter S. Thompson

Monowheels, motorcycles, trikes, and quads.

Terrestrial Motorcycles Human Powered Jet Bikes

The earliest motorcycle figure I know of is the WW1 Dispatch rider from Great War miniatures (available from North Star):

Currently Wargames Foundry only have one motorcycle listed
However they had a range of WW2 bikes in their German range, and given the frequency with which Foundry removes and adds packs, they may well be available again, so we'll include them

Bolt Action (Now Warlord) has a nice BMW with sidecar, British Welbikes, and a Harley Davidson

Artizan Offers Beemers with machine gun mounts as well.

And the trend continues with Black Tree

Eureka have some fairly generic looking vintage Italian bikes available both with and without riders

They also have a selection of modern light vehicles in their Australian SAS range, includeing bikes and quad

West Wind offers another BMW side hack rig, from their Berlin or Bust range.

Westwind also has one of the larger offerings of modern and near future packs with a large variety of riders including biker scum, werewolves, vampires, cops, and more filling out their RoadKill range.

EM4 have a few modern offerings in street bike, police cruiser, and a scrambler type bike

Mirliton has the same bikes as EM4, as well as the rest of the Future Warriors range, which includes choppers, sidecars, and an ATV.

Empress Miniatures have quads in their British SAS range, and some clunky Royal Enfields or some such are available for their Taliban range.

Reaper have a riderless chopper in their Chronoscope line

Call of Chthulu cops, from RAFM

Victory Force have some excellent niche players for the trike market.

MircoArt's Discwold Death on a chopper.

Future Shock has several bikes still available via Prince August.

SuperFigs motorcycle henchman.

The Void and Urban Mammoth ranges seem to have some overlap, and I'm not familliar with the backstory, or what was what originally, but luckily you can find the whole pile at Scotial Grendel as Junkers, VASA, and Triads.

Urban Mammoth also seems to have a current production range of some of the figures offered by Scotia Grendel above, as well as some new additions to Urban War, including this Dune Raider quad.

Warzone's unique offering

Fenryll have a large selection of SciFi/Post-Apoc bikes and riders of different species.

Ramshackle Games has a fairly broad selection of bikes, though some are only available with lower legs pre-attached, which may limit their utility depending on your modelling skills.

Mongoose has some new lawmasters on the go.

Citadel continues to release bikes and trikes for your various marine forces, chaos or space.

While we're in that world. . .
Where would market leaders be without knock offs? Puppets War leads the way with the sincerest form of flattery.

Of course if a thing like that can be done, it can be overdone. To remind you of that, is Scibor, as usual.

Why should the power armored soldiers have all the fun? How about the standard human space grunt? Curious Constructsis looking out for the colonial soldier on the go.

What about tracked bikes?Spartan Gamesoffers the treadcycle.

MadRobot is on the treadbike bandwagon as well.

And Zinge Industriesoffers an armoured treadcycle / tankbike thing.

Infinity have some futuristic designs for you

Mutton Chop has some creative faux-historical offerings.

If near future is more your thing, Iron Wind Metals are still producing some of the ShadowRun miniatures.

Let's talk monowheel, shall we? Antenociti's Workshop speaks your language, if thats what you're after.

And so doesTobsen77 for that matter.
Though he also speaks two-wheeled space bike as well.

And if you just want the parts to make your own monowheel figure, look to Kromlech

Ground Zero Games has a few old 25mm bikers.

As we bring the scale down a little, we find another Italian bike, this one a Moto Guzzi from Mirliton, and in 20mm.

Aberrant Games has your post-apocalyptic needs met in 20mm.

Battle Front have 15mm bikes for many of the beligerents of WW2

As does Peter Pig

In 15mm Sci-Fi is your source for Laserburn and HOF figs.

If small is really your thing, Games Workshop has you covered in 6mm for Epic Space Marine/Adeptus Titanicus /Epic 40k

If you're brave enough, give TheEssexGirl a call and maybe you score some of the various bikes Citadel used to produce. I'm sure she'll quote you a reasonable price.

Also on the ebay only list are the Legions of Steel quads (shown with different riders)

Terrestrial Motorcycles Human Powered Jet Bikes