I would say there are two good options:
- NoCD/NoDVD cracks: If you can find them for the game(s) you are interested in, those are the easiest to use. You simply replace your executable (in most cases), and you're done. The caveat is that you have to be careful where you get these from; downloading EXEs of somewhat shady origins is a good way to catch a virus. There are a few sites on the web, though, that are trustworthy. Also, you may not find such cracks for the newest games, or versions including the latest fixes of a game. And as K9387 said, you generally need to keep the old executables to be able to apply patches, and you usually need to find a new version of the crack once you have patched (which may or may not be available, see above).
- You could take a look at Game Jackal, which is a piece of software that will trick your game into thinking the CD it is looking for is present. In order to do that, it creates a small file called a "game profile" that it stores after you have started the game once with its proper CD or DVD. The advantage compared to the above is that there is no security risk, and it works, in principle, with the newest games and fixes. Two downsides there: Some (very few in my experience, but check their website for details) games do not work well with it, and it is not free (but they often have specials, selling the software for as low as 10 bucks).
Other avenues, like CD-isos in connection with virtual drive programs are less reliable in my opinion. There can be general compatibility issues with the virtual drive programs, and many newer games by now also check for the presence, and balk at those.