If you're scanning a lot of paper documents and you don't need to scan film, then one of the less expensive Epson scanners are a good choice. They're very fast, and will save you a lot of time. Make sure you get one with a USB 2.0 interface.
If you're scanning mainly film (slides or negatives) and want a very high quality, then the Nikon CoolScan V ED is an excellent choice. A critical feature when scanning film is the ability to use an infrared lamp to detect dust spots on the film - Kodak calls this "Digital ICE", but it's just an infrared light source. The Nikon scanners use an LED light source for red, green, blue and infrared light, and these LED lights are better at producing pure colors when scanning film.
If you mainly need to scan paper and only occasionally want to scan film, you might look at the Epson Perfection 4490, 4990 or 4870 or the Canon 9950F. All four have a light source in the scanner lid, and all can scan film with infrared light. All of these are also quite fast when scanning paper. The film scans from these scanners may not be quite as sharp as those using the Nikon CoolScan V ED film scanner, but they are pretty close and should be sufficient for most uses.
In fact the Epson 4870 is a very good all round choice for scanning documents, prints and film negatives and slides and comes complete with Digital ICE software as well as supporting the impressive all singing and dancing Silverfast software.
Another scanner worth mentioning is the Plusteck. This is a budget priced scanner and if you are not looking for the highest possible quality, then this model currently priced at around £130 is a very good choice. Don’t get me wrong. This scanner produces good results but not quite as good as the well known marques already mentioned. This scanner can only scan film.
However, it does have one excellent feature that none of the others appear to incorporate at the moment - a light box on the top of the scanner unit for viewing and editing your slides and negatives. Don’t underestimate this feature. It allows you to view and sort your slides or negatives without using a separate viewer.