I can't believe no one has answered this. Do a search for under the sink water filter. I think that's how I found some great recommendations. I may have searched on google by putting in that phrase and gardenweb.

I put in one of the cheaper recommendations, a two stage Smart Water from the big box store.

It depends mainly on what you want to filter -- how bad your water is and what you want to filter out.

Other considerations are how much room you have for the filter, and the change of pressure at a given flow rate.

In my own case I wanted general filtering and chlorine abatement, had lots of under-cabinet space, and wanted the smallest flow restriction possible because I was putting the filter on the cold water for my prep sink. From
WaterFilters.NET I got

20-BB 1" Whole House Water Filter System
RFC-20BB - Pentek RFC-20BB Carbon Filter (20" x 4.5")

which has worked out well. WaterFilters.NET has a good range of choices.

I've posted on my GE under sink inline water filter many times. It's cheap, easy to install, water tastes great, filters are conveniently found at the orange box store where I bought the unit, and it's very low-tech. Below is a link to an internet search with threads discussing these types of filters, including my comments. Good luck!

Here is a link that might be useful: Previous water filter posts on Internet search