About.com have sections on hundreds of topics – some good and some bad. The Spanish section, run by Gerald Erichson is a site I visit fairly regularly. There is a lesson section, with new articles posted regularly, and an active forum. Lessons are pitched at a variety of levels, and often aim to clear up areas of confusion such as multiple meanings in English for a Spanish word or vice versa. There are also good lessons explaining many of the finer points of Spanish grammar.
There are also some useful email lists to subscribe to, such as the Spanish Word of the Day list and a list to advise when new lessons are available (which is ofter).
There are some quirks to the site, such as the way example sentences seem to be taken from literature or newspapers – this gives an accurate usage, but taken out of context the phrases can sound a little strange. For example, to illustrate the verb 'nadar' (to swim) he uses a sentence about the discovery of a boy’s body who disappeared swimming in a river.
If the site has a downside, it is that it can feel a little dry and classroom like compared to the real everyday Spanish of Ben and Marina for example. Having said that, the nitty-gritty details of grammar and vocabulary are important if you want to master a language.