Celextel's Online Spiritual Library has translations of more Upanishads than I have have found elsewhere (108 of them!), including many that I have not even heard of before. There are also translations of the Gita, Brahmasutra, Panchadasi and a number of works by Shankara.
The Sanderson Beck Foundation has lots of background information to the Upanishads and translations of a number of them.
Here, you can download a pdf format file of 11 of the major Upanishads, translated by Swami Nikhilananda. Recommended (but there is no Sanskrit text).
sankaracharya.org also has Swami Nikhilananda's translations as well as translations of the Vedas and Shankara's works and bhASya-s on the gIta and brhama sutra-s.
Max Muller translations may be downloaded here.
Vidya Vrikshah has word-by-word translations, together with the original Sanskrit for a number of the main Upanishads and other scriptures as well as presentations on the main concepts of Vedanta and discussions of music and poetry. (Note that these will be found in the 'Presentations' section and you may need to download and install the fonts from the 'Software' section before you can see the script correctly.)
You can search eleven of the major upanishads for key words (in English) at WORD.
See also the Recommended Reading page for Upanishads.