The conjugation of verbs is one of the trickier things to do in foreign languages. Fortunately, there's a lot of great resources out there for virtually any language you might be wanting to conjugate a few verbs in.
Is it, He eats the apple? Or maybe, he was eating the apple. Or maybe, he will eat the apple. Or maybe he's not eating the apple at all, maybe he takes the apple, or maybe he has stolen the apple.
And then... there's all those other languages. Il déteste ce pomme? Aber vielleicht er liebt den Apfel...
The conjugation of verbs is one of the most critical parts of coherent communication, and it's (in)famously difficult. Students of foreign languages often memorize frustratingly large tables of exceptions to the myriad of rules for each and every tense. Mercifully, along with the advent of the internet also came the proliferation of websites to help out a bit, no matter what the language.
Verbix
Arguably the most best option, Verbix.com is a non-profit site that includes useful features for conjugating verbs in a hundred-odd languages, from Afrikaans to Vulgar Latin (Eastern.)
The main feature is the online conjugator itself, where you plug in the infinitive of the verb you wish to conjugate, and receive an impressive, if somewhat intimidating, array of the verb's various conjugated forms. It's a fantastic tool, whether you're only just learning a language, or a fluent speaker who can't quite remember where to put that accent in the imperfect subjunctive.
They've got other cool features too, including a “verb finder” and “language guesser” that will attempt to puzzle out the language from any input you provide. Their VerbWiki provides some background for the languages and also some of the logic (or lack of) behind the verbs. They also offer free and paid versions of their software for download.
Be warned: this site does not instruct you on how to properly utilize the verb once conjugated within the context of a sentence structure. Nor does it inform you, for example, of the substantial semantical differences in a subtle change of helping verbs. Verbix, like the other verb conjugating sites featured in this review, is one of many tools to be utilized in communication in foreign languages.
About
Though verb conjugation is not the main feature of About.com, it does offer excellent free verb conjugation services. There's not just verb conjugation here, however. About.com also features articles on how to use these conjugated verbs within the context of actual sentences. For many languages they also offer “deconjugation” and translation services, which serve as outstanding supplements to whatever you're trying to accomplish. Here is the link to the page where you can access the French verb conjugator
However, while About.com is excellent for mainstream languages, they do not delve into the more obscure and less-spoken as Verbix so does. Also, because verb conjugation is not the main feature of About.com, you have to search a bit to find the conjugating services.
Wiktionary
If you like Wikipedia and the philosophy underlying it, a sister site, Wiktionary.com, might be a great place to get involved. Though the general dictionaries and pronunciation guides are fairly well developed, the verb conjugation index is still in the embryonic stage, with only fledgling guides for the rules of conjugation verbs in different languages. This project has real promise however, so keep an eye out!
Other Sites
Though the previously mentioned sites are really the highlights, there's a lot of other sites out there that offer conjugation services either on the side or as their main feature.
Allverbs.com is another up-and-coming site with a lot of promise, with conjugating features in Spanish, French, German, Italian, English, Portuguese, Dutch, Finnish, Latin and Romanian. The support for existing languages is fairly thorough, and promises to improve.
Another good site with support in French, English, Spanish and German is Reverso.net's verb conjugator, with easy links to associated translations and grammar articles.
Language Specific Conjugators
There are also sites that are tailored for use in a particular language, which are worth looking for if none of the above suit your needs. French in particular has particularly good verb conjugation support on the web, with two extremely good sites in conjugationfr.com and verb2verbe.com.