Speaking of Big Bang Theory, it was already announced that an upcoming episode will feature Summer Glau (Firefly, Sarah Connor Chronicles) as herself. This is kind of a “semi-crossover.” Full-fledged crossovers, where characters from one show turn up on another, are still popular, as ABC has proven with its Grey’s Anatomy/Private Practice crossovers. And the Crossover/Spinoff Master page can give you a full history of “true” crossovers. But sometimes the actor from another show turns up playing his or herself, rather than the character. That doesn’t count as a crossover because the two shows don’t exist in the same “universe,” but really, it is a crossover, since the cross-promotional effect is similar.
Though often it’s less about cross-promotion than it is about a plot that calls for a celebrity to appear; the producers may go to another show on the same lot and borrow one of their actors. In this case, Glau’s show, Sarah Connor Chronicles, is a Warner Brothers production, just like Big Bang Theory. And of course, whenever a show does an episode where the characters go to Hollywood, there will be a cameo from a star who appears on the same network or works for the same production company; remember the Seinfeld gang going to L.A. and meeting Corbin Bernsen and George Wendt, or the time the guys from The White Shadow met Ed Asner from Lou Grant (on the same lot and the same network).
The other kind of guest appearance which I always sort of considered a crossover was when a regular from a current series appears in a guest role on another show. This isn’t a crossover in any strict sense, because the character isn’t the one they play on the other show, but it is a crossover in the sense that the network can use this appearance to link the two shows: tonight, [Guy X] from [Show Y] guest stars on [Show Z]. Like the time NBC had Julia Louis-Dreyfus play a guest part on one of their terrible Seinfeld imitators, The Single Guy: she wasn’t playing Elaine, but the point of having her make the appearance was so they could say they had the person who plays Elaine on their show. Of course, sometimes this kind of guest appearance is just the actor taking a week off from his/her show to do another one, like when Sarah Chalke was borrowed from Scrubs to replace Alicia Silverstone in a guest-star arc on How I Met Your Mother. I just think of it as a crossover if the actor is a regular on a show that is clearly much more popular than the one he/she is guest starring on.
(Having a character cross over, on the other hand, can go either way: the character from the less-popular show can visit the big popular one, and that helps just as much as the time The Simpsons made their cameo on the failed spinoff Wiggum P.I.)