1a.  Marriage Index:  Massachusetts, 1633-1850 (Broderbund CD-231):

Webb, Michael

Siers, Rebecca

Aug 25, 1784

Danvers

Essex Co.

1b.  LDS.  Family Search: Internet Genealogy Service: International Genealogical Index (IGI).

Michael WEBB Spouse: Rebecca SIERS Marriage: 25 Aug 1784, Danvers, Essex, Massachusetts Source: Vital records of Danvers, Massachusetts, to the end of the year 1849.

Michael WEBB Spouse: Rebecca SIERS Marriage: 25 Aug 1784, Beverly, Essex, Massachusetts Source: Vital records of Beverly, Massachusetts, to the end of the year 1849.

2.  Census Index:  U.S. Selected Counties, 1790 (Broderbund CD-311):

1790	Webb, Widow	MA	Suffolk Co.	Weymouth Town	p. 209
1790	Webb, Widow	MA	Suffolk Co.	Weymouth	p. 669
1790	Webb, Mrs.	MA	Suffolk Co.	Boston Town	p. 188
1790	Webb, Mrs. ???	MA	Suffolk Co.	Boston	p. 44

I could find no Michael WEBB in Massachusetts.  I suspect these are two widows, not four, and that Broderbund got their records from two sources, hence the duplication (it is not unusual for different sources to have different pagination).  The only WEBBs in Danvers were Nathaniel and Patty, but Nathaniel could be his father (see LDS IGI record below). 

3.  Census Index:  U.S. Selected Counties, 1800 (Broderbund CD-312).  Please see my WARNING regarding use of this database (the numerical data have the genders switched). 

1800

Webb, Michael

MA

Essex Co.

[town not given]

p. 397

11110-2101000

These data should read:

21010-11110-00

which indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
2 males	9 or under	1790-1800	= Son C = Son B
1 male	10-15	1784-1790	= Son A
1 male	26-44	1755-1774	= Michael
1 female	9 or under	1790-1800	= Daughter C
1 female	10-15	1784-1790	= Daughter B
1 female	16-25	1774-1784	= Daughter A
1 female	26-44	1755-1774	= Rebecca

Michael's absence from the 1790 census raises the question of whether these are our subjects, but then, it is not so unusal to get skipped in the census.  Then again, if they married in August of 1784, it's a tad unlikely, though hardly impossible, that they had a child before the end of the year.  And if this is our couple, both the husband and wife are old enough to have had a prior marriage with children born before 1784.  There is also a Nathaniel in Essex Co.

4.  Census Index:  U.S. Selected Counties, 1810 (Broderbund CD-313):  Michael not found, but there is a Nathaniel (and a Nathaniel, Jr.) in Danvers.

5.  Census Index:  U.S. Selected Counties, 1820 (Broderbund CD-314):  Michael not found in Danvers, but there is a Nathaniel (and a Nathaniel, Jr.) in Danvers.

6.  1790-1870 Census Index (online at Ancestry.com):

1820	WEBB MICHAEL	MA	Essex Co.	Salem, Northfield	p. 76
1830	WEBB MICHAEL	MA	Essex Co.	Salem	p. 456
1830	WEBB MICHAEL JR.	MA	Essex Co.	Salem	p. 375

Hard to say whether this could be our subject...  WEBB is a very common name.

7.  LDS.  Family Search: Internet Genealogy Service: International Genealogical Index (IGI).

Michael WEBB Birth: 12 Jan 1757, Woburn, Middlesex, Massachusetts Father: Nathaniel WEBB; Mother: Amy Source: Woburn records of births, deaths, and marriages. Edward Francis Johnson, 1856-1922.

It's possible this is our subject, but again, WEBB is a common name.