Some artists will use the text itself (as a non-intrusive color, something that goes with the drawing and doesn't take away from it). My watermark(s) I made in Adobe Photoshop 7, changing the blending mode to "soft light" so it's slightly opaque, but still clearly visible.
You can also use SAI, or Microsoft Paint to add simple text like Goldenwolfen
does on her artworks. The biggest thing will be to have it noticeable
without it being intrusive
, stealing the attention away from your art.
Or you could use GIMP, which is a free program. I don't know if it has the text feature, but you could make it more personal by [clearly] writing your own name. Some artist will even have a drawing, or "stamp," with their watermark.
The biggest, and I think the most important thing (even over the text color), is the transparent background. Lets say you have a black stamp. Okay, so you want to put it on one of your paintings... but there's a white area around it that you can't get rid of! That's the importance of the transparency.
In case you want it, here's the GIMP download