Other simple add-ons that are still supported are:
1. Extra voices - There are many of these available, some free, some paid for. You'll only find the "real / recorded" type of voice though. there have never been any "computer" / "textto speech" voices available apart from the official ones.
2. Map colour schemes - You can customise the colours for each road type, and background feature etc.
Usually available free, and there's even a program to help you make your own (the "Colour scheme Editor")
3. Car cursors - You can add a small picture of your own car to replace the blue arrow on the 3D-view driving screen if you wish
4. Start-up and shut-down images
5. Custom menus - I can't remember if the Start 45 has a full or "simplified" menu structure, but i it's the cut-down version, it's possible too add a custom menu file to restore some of the missing features. But as the Start 45 is not available in the UK, I've no way to check what menu options are already on it.
6. Speed camera databases - usually on a subscription. Often much better than Tomtom's own service (more accurate, more frequently updated), and you can also have spoken warnings giving you the speed limit in force as you approach a camera. Installed like any other POIs, you just have to set up the warnings in the TT menus.
As well as sites like this one, items #1-5 are also available from TomTom's "Home" PC/Mac utility program, but I'd emphasise it's not the ONLY place to get stuff from. Try a Google search or ask here for anything specific.
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"Real" add-ons, that gave extra functionality such as trip logging, height display, music and video players etc. all used to be very popular, until a few years ago TomTom decided they didn't want people customising their kit any more and withdrew support for the "software developers' kit" that's needed to create extra apps.
So they have mainly died out as they stopped working after one or another software update.