I use a Cokin P set. Very satisfactory for entry level or until you feel like investing larger $$$.

One thing you will need to keep in mind- The very wide lenses (10mm +/-) are likely to suffer some vignetting when used at full width with some holders. I 'modified' a Cokin P holder by removing the slides for all spaces other than for one filter and had to take the corners out. Works a treat, can only use one filter at a time though.