Many companies in the US do not freeze hiring.
Well -of course- hiring varies by industry and arena: You don't really think that construction is busy hiring in December, or retail sales in March, do you?
Take a look here for historical numbers by month: http://www.indeed.com/jobtrends/industry
The differences are easy to spot when you look at, say, Jan 2012 vs Sept 2012, even though the general patterns are similar.
There's a nice analysis on monster:
"Major hiring initiatives may follow close on the heels of the
holidays and summer. "The big months for hiring are January and
February, and late September and October," says Testa. "Job seekers
who make contact right at the start of these cycles have the best
chance of being hired."
You'll also have to consider the company's arena and their fiscal year; for example, a company that survives through online sales is likely to add staff some months before the holiday sales hit, to tweak their site for the reality that most retail-type sales occur during the 6-week holiday period (this year, it's down to less than 4 weeks, though, which should make for possible poor retail performance.
Companies will also hire for the coming fiscal year, which often means that opening are approved in December -- but so few people are in the office in December that the hiring usually starts at the first of the year.
In addition, it's very likely -- but not guaranteed -- that when a company is adding jobs in one area, they're adding in another; i.e. expanding IT jobs will usually suggest expanding accounting, sales, marketing, etc.
More to the point, though, you're going to find that this isn't consistent even with a given company; it depends on more factors than can be generalized. But you can view a few years' worth of job creation history, by segment, at http://www.bls.gov/web/empsit/ceshighlights.pdf
I suggest you do not send your resume during a hiring freeze. Resumes tend to get "stale" and once they have open positions, they will tend to look at the new resumes first. There are some exceptions where companies will enter resumes into a database, and actually search the database when they have open positions; however, that's not as common as you might hope, and almost all new positions are going to be advertised in some way.
If you do submit your resume to a company rather than for a targeted job, do so during the start of one of the more likely hiring periods ... i.e, the first few weeks of January, and right after Labor Day, can work well if new jobs are being created. After Labor Day, you'll also have the advantage of any summer hires going back to school, leaving openings in companies that hire students and new grads.
Oh, and while you're waiting for that optimal time to submit your resume, look for specific job openings, and get your on-line info up-to-date and appealing; as a contract worker, I get a lot of contacts from linkedin and careerbuilder, without much extra effort, mostly because my particular work experience contains the right keywords.