Prowl is a new Growl client for the iPhone. Notifications can be forwarded from your Mac or PC and show up as push notifications on your iPhone (or iPod Touch).
Prowl originally only worked with Growl on the Mac, but the latest (Beta16) release of Growl for Windows adds Prowl support.
***UPDATE 2:*** - (7/13/2009) - There is a new beta version of GfW available that includes the ability to only forward to Prowl when you are idle or away. You can download it from this link, but note that the Prowl API has changed and thus any Prowl forwarding will need to be reconfigured. If you install the Beta17 and Prowl notifications stop working, you will need to delete the Prowl forwarding entry and re-create it using your Prowl API Key (available from the Prowl website).
***UPDATE:*** - There was a bug in GfW that caused the priority of forwarded notifications to be incorrectly set. This bug was making it appears as if the Prowl preferences to limit notifications by priority were not working. I have fixed this bug and re-uploaded the Beta16 code. If you have already updated, please update to the even-newer v184.108.40.206.1 - thanks
Using Prowl with Growl for Windows
Prowl is built into Growl for Windows, so there is nothing extra to download. (Requires GfW v2.0b16 or higher)
On the 'Network' tab, make sure the 'Forward notifications to other computers' box is checked. Click the green + button to add a forwarding destination. In the window that appears, click the iPhone icon to configure Prowl:
Description: a friendly name for the iPhone (also helps differentiate if you want to forward to multiple devices)
API Key: your Prowl API Key
Only forward when priority is at least: lets you select which notifications get forwarded based on priority. selecting '[Any Priority]' will forward all notifications regardless of priority
Click the Save button and the newly-configured iPhone should show up in the list of forwarded computers. Notifications received from that point on will be forwarded to Prowl.
This is a brand-new feature, so if you have any suggestions, encounter any issues, or just have feedback in general, let me know in the GfW discussion forums.