Steam's great—I'd go so far as to say it's one of the best things about PC gaming. It's not so great for gamers with solid-state drives, though. The client requires all games to be installed on the same drive, which poses problems for users with low-capacity SSDs and/or huge game libraries.
Or at least, the client used to have that requirement. As Rock, Paper, Shotgun reports, the latest Steam beta has a hidden user interface that lets you create new installation directories, even on other drives. You're then free to install different titles to different drives—like, say, frequently played multiplayer shooters to your SSD and other, more casual titles to an auxiliary mechanical drive.
The instructions are pretty straightforward. Rock, Paper, Shotgun has documented them in full, complete with helpful screenshots. (The procedure was apparently uncovered by a fellow named RJacksonm1 over on Reddit, so props go to him, as well.)
To sum up, you're supposed to join the Steam beta via the client's settings, add -dev to the program shortcut, and then enter a command (install_folder_ui) into the beta client's console. The command brings up a new window that lets you add installation folders to your heart's content. A Choose install folder drop-down should appear next time you install a game. Disappointingly, RPS doesn't say anything about moving games already installed—worst case, you can probably just use Steam's backup feature to reinstall them to a new location.