(delurking -- hello everyone!)
Howdy!
There are several spacesaver options...depending on how much space you want to save (AND, how much space you willing to make in your wallet).
As a former lurker, you are likely familiar with most of what I am about to share...but here goes.
The ultimate spacesaver, of course, is the HHKB line of keyboards. The best available is Professional 2, which has capacitive keyswitches and a built-in two port hub. But it will also run you close (or over) $300...depending on where you purchase from. I couple of good sources to purchase from are:
http://benippon.comhttp://geekstuff4u.comSeveral people here have the Cherry 4100 (in its various iterations). New they range from $60 - 80, depending on where you buy from. They feature short-throw cherry switches that, IMHO, are downright fun to type on. Google should yield dozens of places you can purchase from. If you are interested in a PS/2, non-windows key version, I'll hook you up with one for the price of shipping. (they can also be found now and again on Ebay) Just PM me your name and address.
If you aren't particular about numpads, the Macally MK96 actually has a smaller footprint than a Model M 84-key spacesaver. This keyboard requires an ADB<>USB adapter (like the iMate), but really feels nice. (take a look at some pics in the keyboard pic thread first as there is some uniqueness to the key layout) These can be found NIB from Ebay for the price of a pack of gum.
Model M "minis" (84-key spacesavers) have been showing up in a variety of conditions on Ebay a lot more frequently lately. There is a current auction for NIB old stock. If you are willing to fork out around $100, that may be enough...depending on how crazy people get. I love the Model M minis, BTW.
If you are truly swayed by the MS (key feel be damned), I'd suggest taking a look at the SMK88. Although there are some blue-cherry-equipped ones out there (I have one and at least two other geekhack members do), they are now only shipping with the linear Cherry switches (black). As much as people hate them here, they are head and shoulders above the typical, crappy rubber dome/membrane switch keyboards that are so common today.
Old Unicomp "mighty mouse" keyboards (and the IBM-branded equivalents) are available in quantity on Ebay on a regular basis. They use what I like to call a "buckling rubber sleeve" and are as fun to type on as the 4100 AFAIC. The added bonus is that they have a built in trackpoint as well!
I'll let others chime in with additional options...I need to go grab some breakfast.