You should use "freeware". I seldom encounter "free ware" and "free software". Note that "freeware" simply means the software that can be used for free, not necessarily meaning that it can be modified (or not) as well.
Just a side note, there are other -wares you might be interested to learn:
Abandonware: software that the original owner no longer offers to the public. Abandonware refers to a product that is no longer marketed or distributed by the author/company that published it. Orphanware is used when the author cannot be located or the company is no longer in existence. Copyright laws apply to Orphanware and Abandonware. The programs are freeware only if they were released as freeware (or given freeware status by the owner at a later date).
Adware: software that displays advertising for other products and/or services (often downloaded from the internet by the software).
Betaware: a preliminary but useable version of what will be the final version of a program. Beta versions are intended for testing by users (alpha versions are test versions for the developers).
CDware: promo CDs included with magazines, books or other products
Commercial Software: software that is sold.
Crippleware: free version of a commercial program. More limited in features and functionality than the commercial product. Crippleware has severe limitations. Functionality that is important to the average user has been disabled.
Demoware: Software that is intended to give potential purchasers an idea of how the program works. You may not be able to test or evaluate all features. Not a full version, type of limitation varies.
Donationware: monetary contribution requested (optional)
Freeware: Legally obtainable software that you may use at no cost, monetary or otherwise, for as long as you wish.
Liteware: free version of a commercial program. A useful program that is more limited in features and functionality than the commercial product.
Malware: software that contains malicious programming
Nagware: has a popup (nag) screen, asking you to purchase the software. You must press a button to get past the nag screen.
Orphanware: see Abandonware
Registerware: you must provide personal information via registration in order to download and/or use the program
Requestware: you are asked to do something. Examples: send a postcard or email to the software author, perform a good deed, make a contribution to charity
Shareware: commercial software that can be downloaded, often for free. Payment is required for legal use of the software beyond the trial period. Some authors use the honor system; more commonly, code is included to prevent the use of some or all functions if payment is not made in accordance with the shareware agreement.
Spyware: software that sends information about you and your computer to others (usually without your knowledge and consent). Spyware may also install files on your hard drive without your knowledge.
Trialware: software which stops working after a period of time or number of uses (including software that can be reactivated without payment and software that has some trial features that stop working).
Warez: Software that is stolen, and/or enabled, by illegal means.
Webware: Online software (the functionality of software is provided as an online service).
source: http://www.pricelesswarehome.org/acf/WareGlossary.phpChris