The SweetWater Microfilter ($59) is also a can't-miss filter. Like the MiniWorks, it's now made by MSR (which purchased SweetWater a few years back) so it has the same basic filter, just a slightly different design. It's being marketed as the "light and fast" filter, while the MiniWorks is for someone who's willing to carry a few more ounces and gain some durability.
Neither the MiniWorks nor the Microfilter is a water purifier, however. That means they'll catch giardiasis, protozoa, and that sort of thing, but not viruses. For that you'll need to carry along a chemical treatment. SweetWater Purifier ($9 a bottle) does the trick, without adding any aftertaste.
Everything you need to know about pumps, filters, and chemical purifiers in Outside's 2004 Buyer's Guide .