Just for clarification...
ockhamsRzr mentioned some of the better postbaccalaureate programs for students who have already completed the prereqs. However, if you're trying specifically to raise your undergrad gpa, programs like Georgetown's SMP and BU's MA in Medical Sciences will NOT raise your undergraduate gpa. That's why I'm guessing ockhamsRzr said "either improve GPA or prove in a grad program that you can hang." So it's not the same thing. As a formal graduate program, the GPA will get compartmentalized on the graduate section of AMCAS. However, programs like G-town, BU, Virginia Commonwealth, Drexel, etc. can sometimes make up for less than wonderful undergraduate showings. I just wanted to clarify so you know that these programs will NOT actually improve your undergrad gpa, but they may make up for it.
Also, if you're looking for the most inexpensive way to work on your undergrad GPA I'd say that taking classes at your state school(s) as a non-degree student is the way to go. The problem is that you might be among the last to register for classes, so you might not get the classes you want. And some schools will limit the amount of credits you can take if you're a non-degree student (some do not have restrictions, especially if you're full-time). You'll have to check with your state schools to find out. One way to get around this is to become a degree-seeking student. For example, if you majored in biology, you could try to go for a second bachelor's degree in something like microbiology/immunology, biochemistry, chemistry, etc. There are disadvantages to doing a do-it-yourself postbaccalaureate at your state school (e.g., huge lecture hall classes, may not get advising, premed committee letters), but I think it's hard to argue that there's a cheaper undergrad postbac out there. Some people find such disadvantages to be significant. If that's the case, then it makes sense to go for one of the more known informal of formal undergrad postbaccalaureate enrichment programs. And out of those, Harvard Extension is one of the more inexpensive postbaccalaureate programs out there.
Also, there's no guaranteed linkage in the Boston U MA Medical Sciences program. Chances are if you do extremely well you'll have a shot at BU, but I wouldn't go as far as to say that it's a linkage. But if you do well in the program and the rest of your package is solid (MCAT, clinical experience, volunteering, extracurrics, etc.) you'll probably get into a US allopathic school somewhere. There are very few US allopathic programs that will explicitly say that they have a linkage and usually these have special qualifications (e.g., underrepresented minority, disadvantaged status). However, there are some programs that have explicit, guaranteed linkages to osteopathic schools (I'm quite sure Duquesne has a linkage to LECOM).
If you're looking for a tried and true allopathic linkage, do Rosalind Franklin's MS in Applied Physiology (formerly Finch). They will never say that it's a guaranteed linkage, but pretty much all of the students who do well in the program are accepted to the med school. My understanding is that the students are even told to apply to RFU's med school during the program. One problem is that estimated costs with living expenses is $52k/year. And if you're there for four more years of med school, have fun paying back loans! It's also on probation by the AAMC, but I don't think that will have too much of an impact in the long run. Schools like Drexel and Temple have been on probation and they don't seem to have any trouble now. But anyway, if you complete the program (48 credits in one year, most of your classes with the medical students, and you have to outcompete them = hell) with a decent performance, chances are more than likely that you'll be accepted (unless there's something really wrong with you). It's risky though, because if you do NOT do well, you're basically screwed if you try to get into a US allopathic school in the future. So it's a huge gamble (but it's a risk many are willing to take).
Like ockhamsRzr's said, many of these programs have been discussed extensively (including BU).
