Files with .2DA extensions are two-dimensional arrays, or tables, or spreadsheets. The Witcher did not introduce any change here — they are in exactly the same format as they were in NWN or KotOR. This means that you can use any one of the tools already available. The 2DA files are plain text, and the columns are delimited by whitespace characters (e.g. Space and Tab) — with small 2DA tables a simple text editor will do. For larger tables however, I recommend some more serious tool — for example, Excel.