A mixed logit model (sometimes referred to as a random parameters logit model) estimates distributional parameters that allow for individuallevel heterogeneity in tastes that are not compatible with a traditional logit framework. Mixed logit models can also provide for additional flexibility as it pertains to correlated random parameters and can be used with panel data.
For more information about mixed logit models, see Wikipedia: Mixed Logit.
Keep in Mind

The mixed logit model estimates a distribution. Parameters are then generated from that distribution via a simulation with a specified number of draws.

The estimates from a mixed logit model cannot simply be interpreted as marginal effects, as they are maximum likelihood estimations. Further, the variation at the individual level means estimated effects are relative to the individual.
Implementations
R
To estimate a mixed logit model in R, we will first transform the data using the dfidx package. Then we will use the mlogit package to carry out the estimation.
# Install mlogit which also includes the Electricity dataset for the example.
# The package dfidx will be used to transform our data
# install.packages("mlogit", "dfidx")
library("mlogit", "dfidx")
# Load the Electricity dataset
data("Electricity", package = "mlogit")
# First, we need to coerce the data to a dfidx object
# This allows for a panel with multiple indices
# For further documentation, see dfidx.
Electricity$index < 1:nrow(Electricity)
elec = dfidx(Electricity, idx = list(c("index", "id")),
choice = "choice", varying = 3:26, sep = "")
# We then estimate individual choice over electricity providers for
# different cost and contract structures with a suppressed intercept
my_mixed_logit = mlogit(data = elec,
formula = choice ~ 0 + pf + cl + loc + wk + tod + seas,
# Specify distributions for random parameter estimates
# "n" indicates we have specified a normal distribution
# note pf is omitted from rpar, so it will not be estimated as random
rpar = c(cl = "n", loc = "u", wk = "n", tod = "n", seas = "n"),
# R is the number of simulation draws
R = 100,
# For simplicity, we won't include correlated parameter estimates
correlation = FALSE,
# This data is from a panel
panel = TRUE)
# Results
summary(my_mixed_logit)
# Note that this output will include the simulated coefficient estimates,
# simulated standard error estimates, and distributional details for the
# random coefficients (all, in this case)
# Note also that pf is given as a point estimate, and mlogit does not generate
# a distribution for it as it does the others
# You can extract and summarize coefficient estimates using the rpar function
marg_loc = rpar(my_mixed_logit, "loc")
summary(marg_loc)
# You can also normalize coefficients and distributions by, say, price
cl_by_pf = rpar(my_mixed_logit, "cl", norm = "pf")
summary(cl_by_pf)
For further examples, visit the CRAN vignette here.
For a very detailed example using the Electricity dataset, see here.