Earlier this month, the Office for Democratic Institutions and Human Rights issued this report on the French presidential elections (April-May 2007), which discusses some of the campaign finance regulations.
One feature of note is that the only existing penalty for non-compliance with the spending limits is the sacrifice of post-electoral reimbursements, a combination which makes for a system to the US public funding system. There are important differences, of course. The spending limits in the French system are mandatory (though with weak penalties), in the US they are conditional on accepting the money (up front). The US subsidy is pre-electoral (except for candidates from new political parties), while the French is post-electoral. And the US system is effectively dead at this point (though after having raised less money than his rivals, Democratic candidate John Edwards recently said he would accept the matching funds), while the French system seems to be alive and well.
This last point I gather from the fact that at least losing candidates did not exceed the spending limits for the first round (16,166,000 Euro, Le Pen and Bayrou spent about 10M each) and neither Sarkozy nor Royal exceed the two round limit - the second round limit is near 11M Euro, and they each spent about 21M each (the reports I have seen do not separate their expenditures by round). The French reimbursements are up to half of the first round spending limits for candidates who get more than 5% of the vote; for candidates with less than this, the potential reimbursement is one-tenth of this. At the current rate in the US (according to the CNN article linked above), primary candidates can get about US$21M in matching funds, and general candidates get US$84M.
But - to return to the comparison - since the French system gives no punishment for exceeding spending limits other than the sacrifice of the public subsidy, the similarity remains that both systems make spending limits optional and public funding conditional.
Another curious set of rules - with no comparison to the US - are those regarding campaign posters: they to be approved by the CNCCEP in advance, that they can only be placed in places designed by local officials, and most importantly, “they must have the same content throughout France.”
Lastly, the rules regarding the media and broadcasting in France seem complex, and they include prohibitions on selling candidates TV time and requirements that each “broadcaster must provide a set amount of time for each candidate to broadcast pre-recorded election spots.” In their report, the Office recommended simplifying these rules.