You might want to decide how you want to make your levels before you get into tools requirements. You have basically three paths you can pursue: BSPs, model levels (usually X format), and matrix levels. Of course, you can combine some of these methods.
Some people are quite handy with BSPs and use tools like Valve Hammer. Model based levels can be built with any model editor such as JTEdit. Matrix editors do exist, I don't know any off the top of my head. It possible to make levels based on height maps as well.
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TAZ