As a collector, it's moments like this that make you feel like you know what you're doing. Here's my newly swelled collection of Astromech Droids. Among the many regular off-the-peg acquistions, there's some pricey Star Tours exclusives, a member of a Hoth four-pack and, best of all, the exclusive red droid from the Naboo Royal Starship.
I don't have every incarnation of R2 himself, nor do I intend to go after them. I thought I'd want him as a drinks server, but the tiny arms looked so chintzy and badly made that I passed. There's also the chromed collectible R2, but nah.
There is one missing non-R2 that I know of; that first wave of Sith figs featured an Astromech, and I never got him. I got a Sith Astromech they put out recently (reddish brown dome, lower left), but I missed that first one. I also know there's a new R5-D4 out soon, if not already (I do have that first awful missle-firing R5, but screw that thing).
And it's entirely possible there's more out there I missed.