Recently I have been contributing to the Open Plaques project which documents "the historical links between people and places, as recorded by commemorative plaques". Essentially it is a database of plaques from all over the world contributed by people who pass by a commemorative plaque of any type.
The idea is a simple one - if the plaque is not already listed on Open Plaques, it is easy to add one. A photo is ideal, though it should be readable. Some plaques are quite high up and you need a reasonable camera to get a good shot. Each plaque entry should have the details added to the site - a simple transcription of the text (in all languages) should be provided as well as such details as who posted the plaque, what colour it is, and when it was posted. Quite a few plaques plaques in the Dublin area are already listed, mostly thanks to Dr Michael Seery of DIT (who is a historian as well as an award winning Chemistry Lecturer).
Plaques of any type are accepted - not just the round blue ones that tourism boards put up. So any time I see anything of interest I take a photo and check Open Plaques to see if it is already there (most likely Michael Seery beat me to it). Below are some of the plaques I've added (or were added for me by the good folks in Open Plaques), with links to their Open Plaques page: