Online Qur'ān Study Resources
In all likelihood, this is already a constant reference for most of us. The variety of translations and recitations available is more than sufficient, and the interface is simple.
but especially:
This site is also posted under Arabic grammar resources. It's a great place to go if you want to see exactly how a sentence in the Qur'ān is put together. (Not on the level of tafsīr so much, but in the linguistic sense.)
A program that you can use offline to read the Qur'ānic text and search within it. There are versions for various operating systems, and the software has almost limitless configuration options. Good stuff!
This site can be useful under certain circumstances. In particular, I have made use of the topic index. On this site, each verse in the Qur'ān is tagged with some of the relevant topics discussed therein. So you can bring up, for example, all the verses that deal with Jesus. or
(Also listed under Islamic Texts/Resources.) This site has recitations, translations of the Qur'an, tafsir, and translations of tafāsīr, among many other offerings.