Orange
Orange's prepaid plan is called "Mobicarte". Mobicarte's "Internet Max" option costs 8€ and provides unlimited data for 7 days. The option doesn't includes the use of email protocols (smtp, pop, imap). For email a seperate prepaid plan can be bought. Without a data plan, Internet is charged at 50 cents per minute thus costing a fortune and quickly sucking your prepaid account dry. These data plans are for mobile phones and may or may not work on other gear. Your phone must be provider unlocked so it can work with Orange's service.
To obtain unlimited data, first purchase an Orange SIM card (15€?) and top it off for 30 days or more of service (no more than 15€ needed). Obtain the Internet Max option by either 1) calling customer service, 2) dialing #123# and using the menu system (Multimedia->InternetMax), 3) dialing 220, or 4) orange.fr (recommended if you don't speak french and have temp wifi net access as you can google translate each page). Be sure your phone is set to enable 3G and data services, and to disable mail if you don't want to be charged for it. Note: You may not be able to add the Internet Max option to your account until your registration information has been added to Orange's systems, this may take 24 hours initially.
Likely you will not receive any notification of Internet option activation. You can test whether the plan is in effect by this procedure; 1) check your prepaid credit amount by dialing #123# or create an online account on orange.fr, 2) do a tiny bit of web surfing, 3) again check your credits and compare against the original amount. If no money has been deducted, likely your Internet option is in effect.
Note that Orange only guarantees 500MB of traffic, after which bandwidth gets limited. But so far, the limit does not seem to be enforced.
Store employees are usually unaware of prepaid data options so don't expect prepaid data assistance from them. Many different plans are available for unlimited calling, evening calling, evening data, etc. See their site for more information. Your account is managed on the phone by dialing #123# or calling 220, or by visiting any Orange boutique or affiliated vendor.
Default data rate
Unlimited 3G Speeds, excluding email.
Email is available for 9€ extra per month.
Normal Pricing
SIM card + 5€ credit = 15€
Calls 0.37€ / min to Mobile
Calls 0.34€ / min to Landline
SMS 0.12€
Data 0.50€ (!) / minute (It cost you a fortune and will quickly suck your prepaid account dry.)
Email / GPS 0.15€ / 10k
Pricing Plans
Unlimited internet 1 day 3€ (actually 22 hours from 2am to 12am - watch out for the 2hours in between)
Unlimited internet nights and all weekend 6€
Unlimited internet 1 month 12€
Unlimited e-mail 1 month 9€
Unlimited calling to Orange mobiles and all landlines daytime (7h - 17h) 3€
Unlimited calling to Orange mobiles and all landlines evening (21h - 24h) 3€
Unlimited weekend 10€
Availability
SIM and Mobicarte available in Orange stores and related vendors, and online.
Data plans must be subscribed to via automated phone system by dialing 220, online or #123# menu system.
SIM Sizes
Mini SIM (2FF)
Micro SIM (3FF) (iPhone 4 and iPad)
Tethering
Not allowed (But they dont seem to bother if you do)
SFR
Subscribtion is called "La carte". An activated SIM card is 9.90 euros, and the 'iPhone3G Internet + Mails Illimités' recharges provide data service. These are priced at 3 euro for 1 day, 10 euros for 7 days, and 20 euros for 20 days. Text and calls are charged separately. See their site for more information.
You probably should be able to speak french to use SFR. The support lines are reputedly only allowed to speak french, and the prepaid section of the website is all in french.
Default data rate
Unlimited
Availability
In some - but not all - SFR stores. Generally not available at tabacs. Recharges available online, but the site requires an existing SIM card and a European (smart chip) credit card.
SIM sizes
Mini SIM
==
Tethering
Not allowed