The oddly numbered Windows 10 is just about ready to roll out. The official release date is tomorrow, July 29th at midnight US time (7am in the UK). Those who have opted into the free upgrade from Windows 7 or 8 may already be downloading the new OS. Microsoft are doing a phased release schedule so as not to overload their servers.
This handy Reddit thread should help you determine if you're already downloading Windows 10 and will have access tomorrow or if you have to wait a bit longer. Essentially if there is a a folder named "~Windows ~BT" in your root directory, you're good to go. If there isn't, it's possible the download is still progressing or you need to run Windows update to make it start. Otherwise, you're probably further down the list and will gain access in the coming few days.
Any legitimate Windows 7 or 8 install can upgrade to Windows 10 for free for the next year as Microsoft pushes to keep people up to date. There's a little box next to your system clock, if you want it. Poor luck XP hold-outs, but you probably weren't planning on joining this decade anyway. As for those of you on 7, I'm personally running 8 and have few complaints about it, all of which seem to be fixed in 10, so I'll be heading that way.
Here's what Tim thought of his time with a pre-release version.