Memory cards are the “film” for your digital camera. You don’t want to have a card failure and lose your photos. A reliable brand is important.
There are other good brands but these two are very popular with working professionals due to their high reliability. I have had other brands fail but I’ve never had a Lexar or SanDisk memory card fail. Be sure you get the right kind of card for your camera (CompactFlash, Smart Media, etc.).
One important piece of advice. Eventually, memory cards do fail, even from the best manufacturers. It doesn’t happen often, but it does happen. So it is an excellent idea to have several cards with less memory rather than one card with a huge amount of memory. Four cards with 2 GB of memory are better than one 8 GB card. When I do a major photo shoot, I split it up among several memory cards. If one card fails, I haven’t lost the whole shoot.
The best of the best photography gear, books, accessories, and online photo labs: a series of articles.