Whoops meant to post this yesterday. Anyhow...
The real question is how much do you have to spend? When it comes to the more exotic and unusual wavelengths about the biggest limiting factor is how much can you spend.
If you're looking for something that's not super common but still affordable you can't beat getting either a 638 red-orange or a 520nm direct green diode from DTR's shop. For around $150 you can get the diode pressed into a copper module, a driver pre-set with your specified current and a G-2 lens. You can get the diodes alone for less but honestly unless you're experienced pressing diodes, setting and soldering drivers to the diode and have the tools to do so it's worth it to pay someone else to do it especially when it comes to the smaller 3.8 mm ones. One thing to note is the 300 and 500mw 638 diodes are multimode so you'll get the same bar-like "dot" like from a 445 diode. Once you have your diode/driver combo you just need a host and make sure you wire it up correctly (case negative/positive, etc).
Now if you can spend more that opens things up quite a bit. These ranges aren't set in stone but they're good estimates.
$250-350
Things start to get fun here. Aside from lower powered 473nm DPSS lasers you also get Helium-Neon (HeNe) lasers as well. If you're interested in a 473nm check the Jet Lasers group buy thread or contact Gray or Hak to see what's available. I've been very happy with mine from them.
On the gassy side of things there are two main options; the common 632.8 red and the less common 543.5 green. If you're really lucky you may be able score a yellow with PSU for a good price. I managed to snag one for $350 not too long ago but deals like that are pretty rare. There's also 612nm orange HeNe's as well but they're about as common as a two peckered billy goat. HeNe lasers are fun but don't expect super bright beams that you can see in a lit room. The gas lasers forum is a must if you're looking into gas lasers as there's some great deals out there but also a lot of overpriced crap too. Another place to look is the eBay finds thread over in the deals and discounts forums. Please be respectful of any member who has called "dibs" or indicated they are boding on something.
$400-500
Now things get really fun. This is where you find Argon Ion lasers. Sometimes you get really lucky and can find a working setup for under $250 but those deals are only slightly less rare than the above mentioned billy goat. There's not much IMO that tops a good multiline argon in sheer beauty. Again ask around in the gas lasers forum before buying.
$600+
Once you hit this price point you can get some of the truly exotic wavelengths like 589, 594, and 671 but be prepared to sell an internal organ to get one. It took a log time to coax my wallet out of hiding after getting my 80w 589nm but it is so worth it.
There are other options I didn't list but these are the most common of the "other" wavelengths.
