Ultimate Tag Warrior is a multi-headed hydra of a plugin. It allows you to add tags either through the Write Post page in wordpress in a tag box, on posts using an AJAXy box, and in posts using special syntax from external editors (or internally, if you’d like). From the write post page, you can also get suggestions for tags using the Tagyu service.
Once you’ve got your tags entered; you enter a realm of interesting ways of doing things with your tags. At the complicated end, AJAXy search is included with the download (it may need some customisation to match your theme). At the simpler end, you can automagically have tags included at the top an