It may not be Microsoft Bob, but boy am I happy that I installed Clippy (US$0.99 in the Mac App Store) on my Mac. No, Clippy isn't that annoying "It looks like you are writing a letter" implementation from years past, but something far more helpful.
The new Clippy, the better Clippy, the Macintosh Clippy allows your Mac to remember previous pasteboard entries for those always awkward times when you have to copy not one but two or more separate items after another. You can pull items as you need them directly from the menu bar at the top of Finder.
Clippy is, at the current time, text only -- so you're not going to be able to store images, sound snippets and the like. Also, it doesn't seem to store text formatting along with the core text, which is either a disappointment or the Best Thing Ever, depending on your use case and viewpoint.
I'm really happy with my $0.99 purchase -- and if you do a lot of copy and pasting you may be, too. Clippy was created by Naheed Kausar of Faw_zz. Commenters also suggest the non-App Store (and free!) options of JumpCut or ClipMenu, both of which are listed as still under development. There's another tool that's both open source and for sale in the Mac App Store: Clyppan, which costs $10 in the store (version 1.1) but is also apparently downloadable to build yourself in Xcode (version 1.0).