How can I check if Steam is up?
Steam usually makes downtime announcements on their forums.
Steam is under heavy load right now, because Valve pushed out the Team Fortress 2 Australian Christmas 2011 update 20 minutes ago, and managed to mess up a file in at least one of the server distributions.
Basically, servers can no longer detect if they're up to date and are flooding the content servers trying to update.
(The best part is, they're in denial that this is happening despite a good number of us reporting it on the dedicated server mailing lists)
One of the best ways to monitor outages is to check Steam's graph which displays the number of concurrent users who are logged in:
http://store.steampowered.com/stats/
If the value on this graph drops substantially, or there is a dip, you will know that there are other users who are having issues connecting.
For example, you can see here that there is clearly a service disruption where the graph drops. You can also note that the number of current users at the bottom of the graph is low:
There are other ways to check such as viewing the last page of this thread, or checking this unofficial site, however they are not as reliable as the (semi) real-time graph of concurrent users. These other sources are not always updated in a timely fashion, and may not reflect regional outages which should be visible by a dip on the graph.
Note that this method is only effective for checking content servers, and is not a reliable way of checking whether or not Steam's website is down. Use this tool to check the website's status.
Hopefully, Steam will adopt a better status model in the future, such as GitHub's status page. Updating a forum thread (as they currently do) is woefully bad practice for this sort of thing.