Ok, so I have had a look around the 'net and this is what I came up with. My first suggested site is InterPals Penpals . This site seems to be the most comprehensive of all the sites I looked at. If you can put up with the advertising, which doesn't seem to be too much of an annoyance, the sight is free for all users. It has been that way since it began back in 1998 and the webmaster notes claim that it will remain that way. It has a very large pool of members to choose from and in fact supports not just English language penpals, but penpals in a variety of languages. A brief stroll through the site suggested that this site is well maintained. There is a disclaimer that it cannot be held responsible for everything posted on it by the public, but it seems to be well monitored and does state that it will remove anything that is offensive or innappropriate, along with whoever uploaded that material.
A second website which seems to be quite popular is PenPal.World . This sight claims to have 2.1 million members and it is free, although there is a paid VIP section. The site seems to be for adults and children, and it claims that children can block adults and specific users (eg. the person who tries to connect with you and won't take no for an answer). Users can block entire countries; I'm not sure I agree with this, as I am pro-communication as a way to solve cultural disputes, but I do understand why someone might want to have this option.
This brings another point, and one that concerned me. The majority of sights allow children to join but seem to be focused on adult membership; there didn't seem to be much of a selection focusing on children and young adults. However, I was able to come up with the following websites:
Kids' Park Japan
The site is managed by 32 Japanese students and does not make any claims regarding its membership size. The front page states that it was last updated in February 2007, so I cannot vouch whether or not this is still a live site. But it was near the top of a list of Yahoo! search results. This should indicate that it is a popular site, so I'm going to assume that it is still active.
The front page graphics indicate that this site is for children 13 years old and younger, therefore I didn't venture inside this website. There is a note stating that the Idea Seeker Universe is still open for making friends around the world. You'll have to check this out for yourselves but it looks ok. Anyone who wants to send me feedback about this site, please do.
Make A Child Smile
This site is sadly closed down. It was one of my favourites, but I will be looking for another children's penpal website to replace it.
Circle of Friends PenPal Club
This site is for girls only, but it no longer seems to be online. I will keep researching and update the link if I can find it.
ePals Global Community
This site encourages teachers and entire classes to join, which is perhaps the smartest strategy in maintaining site integrity and keeping out anyone who shouldn't be there. The front page makes reference to international classes engaging in study projects in addition to fostering penpal relationships. It also allows for communicating in languages other than English.
If I have missed out on anything, or if anyone has a sight they know of that they would like to add to this list, I encourage them to write and tell me.