Using a Galaxy Nexus I download an apk and it showed up in the notifications bar when it was done downloading. I went to install but I didn't set option to allow external apps so I did that and I went back to the browser to install the apk and couldn't find it. It wasn't in the notifications anymore and there didn't seem to be a "downloads" option anywhere in the browser. I ended up having to download it again just so it would be in the notifications so I could then get to the apk to install it. And I downloaded it AGAIN to show a friend the issue. Now I'd like to delete all three and know where to go next time.
There should be a "Downloads" app in your app drawer. Open it to view your downloads. Previous versions of Android (up to Gingerbread, AFAIK, possibly Honeycomb) had a "Downloads" option in the Browser itself under
Menu->More->Downloads, but it's possible this no longer exists on ICS.
Alternatively, if you have a file manager they should be in
/sdcard/download, I believe (on ICS this actually appears to have changed to
/sdcard/Download with a capital 'D'). It can vary by device a bit, but generally it should be