2-bit full adder using just thirty six 555 timers

741s are around in books and in EE labs because they're what people used back in the 70s' (and perhaps cause the lab bought thousands back then). Since there was such a huge volume of designs produced using them, you can still buy them today.

No engineer in his right mind* (no disrespect intended: I know you're a student) would use a 741 for a new design today. They are slow, imprecise power hogs, and require bipolar supplies to get them to do anything. Even a jellybean LM324 outshines them.

(*) if anyone can counter this statement with a useful application of the 741 where a more modern and cheaper opamp wouldn't do better, I'd be really interested in seeing it.

[edit] It looks like the 741 has been improved quite a bit since I last looked at it. So I don't need to be so harsh, but I still can't see any good reason to use it for new designs in 2011.

There are better opamps out there, and even some nice uni-polar rail to rail ones which would make for a better classroom experience but the concept of an opamp hasn't changed enough to make what you learn about opamps with a 741 invalid.

The 555 on the other hand, well it could be replaced by an 8 bit internally clocked micro (ATTiny, PIC 12F*) but I've heard that in quantity they cost less than 5 cents so in that regard they are competitive.