That's a pretty big "just"

As a dropbog alternative, I use Live Mesh from MSFT - 5GB storage and possibility to sync more folders.

Regarding the Google offer: It seems to me that the storage can be (currently) used only for photos (picasaweb) and gmail. It doesn't look bad, but I would still prefer to pay price for bandwidth per year. If I have to use this as a backup for all my photos in original res., 20GB is not enough, so the Flickr's unlimited storage for $25 still looks better.

For me, the OK price would be $5-$10 per 20GB upload bandwidth/year.

I'm using SpiderOak which gives you 2GB free, but you can get an additional 1GB for each free referral, for a total of up to 5GB free. You get a 'zero-knowledge knowledge environment' (everything's encrypted on the client-side before upload, and they can't even see the file names or other meta-data). You can use an unlimited number of devices, sync folders between machines, no special 'dropbox' folder is required, share folders with others, and use their web interface (though I'm not sure what the implications of using that are on the 'zero-knowledge environment'...I don't know how they've designed that).

Does Live Mesh support Mac and Linux because if not it isn't so much a competitor to Dropbox.

I just said that I use it as an alternative, since I use Windows. So it is a competitor for PC+Windows users, if you want me to be more exact.

It does support Macs. It's got an API so hopefully someone will build other clients, like for Linux.

Of course they could.

But they'd have to want to be in that market and then to provide a better UI and functionality set than the existing system.

And would they make enough money off of it to be worth investing their time in? Probably not.

they won't; both serve different needs.

dropbox is mainly used by people who don't use webapps to manage their documents and photos.